fix(event-streams): use member name as :event-type header value

[kuhe]

When serializing the headers for an event stream event, the :event-type should take on the member name and not the member target id.

  const headers: __MessageHeaders = {
    ":event-type": { type: "string", value: "textEvent" }, // <-- here, for example
    ":message-type": { type: "string", value: "event" },
    ":content-type": { type: "string", value: "application/json" },
  };

This had not manifested until a recent AWS service in which the two values were not identical for apparently the first time.